A large number of railroad and tugboat diesel engines use silver or silver-plated bearings. As a result, the lubricating oil, which typically requires oxidation stability, wear control, deposit control and alkalinity, must also give acceptable silver wear and corrosion performance. While these properties can be achieved by the use of lubricating oil additives known in the art, many of these additives cause unacceptable corrosion and wear to silver engine parts. Also, typical bearing protection additives, which are effective to protect other material bearing surfaces like brass, copper-lead, bronze, aluminum, are ineffective to protect silver bearing parts or are deleterious to silver (e.g., zinc dithiophosphate).
At present, silver protection is largely provided by the use of lubricants containing chlorinated paraffins or other chlorinated additives. Examples of chlorinated additives used to provide silver protection are described in U.S. Pat. Nos. 4,131,551; 4,169,799; and 4,171,269. However, there is a concern that the use of halogenated additives might cause an environmental problem when disposing of the used oil and oil filters. Thus, there is a need for lubricants which provide silver protection without the inclusion of chlorinated additives.
In view of this need the art has already developed certain chlorine-free or reduced chlorine silver corrosion inhibitor-containing lubricants. For example, U.S. Pat. No 4,734,211 discloses a marine and railroad diesel engine lubricating oil composition containing certain polyhydroxy esters as silver wear inhibitors. These patents also disclose lubricating oil compositions containing a mixture of these polyhydroxy esters and chlorinated paraffins. U.S. Pat. No. 4,820,431 discloses a method for reducing silver wear in marine and railway diesel engines using similar lubricating oil compositions. Unfortunately, these polyhydroxy esters are expensive, they are incompatible with some oils, and they can cause copper-lead corrosion.
U.S. Pat. No. 4,171,270 discloses lubricating oil compositions containing a sulfurized overbased calcium alkylphenolate and a sulfurized naphthenic base oil-containing composition having a sulfur content of from 1 percent to 6 percent by weight. Unfortunately, the sulfurized naphthenic base oil can cause oxidation and viscosity increases in newly required standardized oxidation tests.
U.S. Pat. No. 4,282,107 discloses a diesel crankcase lubricant composition containing a non-CO.sub.2 blown 2:1 calcium hydroxide overbased calcium salt of a sulfurized alkylphenolate, an alkenyl succinimide and a pour point depressant in a mineral oil base. These compositions are also thought to have silver wear and corrosion properties, but the described product is expensive, can have high lead weight loss, and can have high oxidative base loss.
U.S. Pat. No. 4,871,465 discloses lubricating oils containing as a silver protectant (a) a sulfurized olefin, sulfurized fatty acids, sulfurized hydroxyaromatics, 1,3,4-thiadiazoles, and dithiocarbamates and (b) the reaction product of a saturated aliphatic dicarboxylic acid with an optionally substituted amino guanidine. Unfortunately, these protectants are expensive, they are incompatible with some oils, and they can cause copper-lead corrosion.
U.S. Pat. No. 4,948,523 discloses a chlorine-free silver protective lubricant composition, using the reaction product of a carboxylic acid and an amine. Unfortunately, these protectants are also expensive, they are incompatible with some oils, and they can cause copper-lead corrosion.
Other organic compounds have also been disclosed as providing silver protection. U.S. Pat. No. 4,278,553 discloses a railway diesel engine lubricant containing a silver corrosion inhibitor comprising a benzotriazole compound present in concentrations from about 0.5 to 2.0 wt % and U.S. Pat. No. 4,285,823 discloses a diesel lubricant composition containing a silver corrosion inhibiting compound of an N-substituted 5-amino-1H-tetrazole. Unfortunately, these protectants are also expensive, they are incompatible with some oils, and they can cause copper-lead corrosion.
Therefore there is a need for a silver corrosion inhibitor that is inexpensive, compatible with most oils, and does not cause copper-lead corrosion.